If you haven’t tried tuning your guitar by ear yet I recommend doing that first so you can truly appreciate this method of tuning. Besides, this is just one giant experiment anyway and getting your ear in tune is a part of becoming a better musician.
So… now that you’ve tried to tune your guitar by ear, here’s how everyone else is tuning their instruments and its the easy, lazy way. It will also invoke confidence that you really are in tune (at least for that moment the meter lines up). Its a digital tuner.
Power up your tuner and set it somewhere close to your guitar. 
Your tuner uses a microphone that “listens” for frequencies. On auto mode it will actually try to determine which note you are trying to tune to and will indicate that in some way. Start by tuning the sixth string, Low E. Turn the tuning peg on your guitar until the display indicates “E”. Now that you are close to the correct note its time to fine tune. On most digital tuners you will see 2 arrows > <. If so, you want to tune up or down in order to make both arrows light up. If your tuner has a meter style display, make the needle go to the center.
Boss makes a lot of great tuners. I have owned both shown on this page and have been very happy with their performance. There are also many iPhone apps available. I am using TuneORama iPhone app which costs $2.99.
I recommend the Boss TU-2 if you play electric guitar and especially if you play live. You can use the pedal to mute your guitar while tuning.
Happy playing and stay in tune.
